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10. HALOGEN DERIVATIVES Can you recall ? CH3 - CH2 - X CH2 = CH - X Identify the functional group (Haloalkane) (Haloalkene) in the following compounds....

10. HALOGEN DERIVATIVES Can you recall ? CH3 - CH2 - X CH2 = CH - X Identify the functional group (Haloalkane) (Haloalkene) in the following compounds. HC ≡ C - X X i. ii. CCl2F2 Br (Haloalkyne) (Haloarene) (Benzyl bromide) (Freon - 12) b. On the basis of number of halogen atoms, iii. Cl iv. Cl - CH = CCl2 halogen derivatives are classified as mono, Cl Cl di, tri or poly halogen compounds. (Westrosol) X Cl Cl CH3 - CH2 - X Cl (Hexachlorobenzene) Monohalogen compounds v. Cl CH3 - CH - X CH2 - CH2 X Cl Cl X X X X Cl Cl Dihalogen compounds Cl (Benzene hexachloride) (Hexachlorocyclohexane) X CH2 - CH - CH2 X The parent family of organic compounds X X X is hydrocarbon. Replacement of hydrogen X atom/s in aliphatic or aromatic hydrocarbons Trihalogen compounds by halogen atom/s results in the formation of halogen derivatives of hydrocarbons. We will consider classification of mono halogen derivatives in more detail. In this chapter we will study halogen derivatives in a systematic way. 10.1.1 Classification of monohalogen compounds : Monohalogen compounds are Internet my friend further classified on the basis of position of Find out the structures halogen atom and the type of hybridization of of two thyroid hormones T3 carbon to which halogen is attached. (triiodothyronine) and T4 (thyroxine). a. Alkyl halides or haloalkanes : In alkyl How do these help our body ? halides or haloalkanes the halogen atom is 10.1 Classification of halogen derivatives : bonded to sp3 hybridized carbon which is a part of saturated carbon skeleton. Alkyl Halogen derivatives of hydrocarbons are halides may be primary, secondary or tertiary classified mainly in two ways. depending on the substitution state of the a. On the basis of hydrocarbon skeleton to carbon to which halogen is attached : (Refer which halogen atom is bonded, the halogen to Std. XI Chemistry Textbook, section 14.3). derivatives are classified as haloalkanes, haloalkenes, haloalkynes and haloarenes. 210  R d. Vinylic halides : In vinylic halides halogen R - CH2- X R - CH- R R − C − X atom is bonded to a sp2 hybridized carbon X R atom of aliphatic chain. Vinylic halide is a haloalkene. Primary Secondary Tertiary halide halide halide X (1 halide) 0 (2 halide) 0 (3 halide) 0 CH2= CH - X b. Allylic halides : In allylic halides, halogen e. Haloalkyne : When a halogen atom is atom is bonded to a sp3 hybridized carbon bonded to a sp hybridized carbon atom it is atom next to a carbon-carbon double bond. a haloalkyne. X CH ≡ C - X CH2= CH - CH2 - X f. Aryl halides or haloarenes : In aryl halides, halogen atom is directly bonded to c. Benzylic halide : In benzylic halides the sp2 hybridized carbon atom of an aromatic halogen atom is bonded to a sp3 hybridized ring. carbon atom which is further bonded to an CH3 aromatic ring. X X CH3 CH2 - X C X CH3 Table 10.1 Names of some halogen derivatives Formula Common name IUPAC name CH2Cl2 Methylene chloride Dichloromethane CH3CH2Br Ethyl bromide Bromoethane CH3CH(Cl)CH3 Isopropyl chloride 2-Chloropropane (CH3)2 CH - CH2Br Isobutyl bromide 1-Bromo-2-methylpropane (CH3)3 C Br Tert-butyl bromide 2-Bromo-2-methylpropane (CH3)3 C CH2Cl Neopentyl chloride 1-Chloro-2, 2-dimethyl pro- pane CH2 = CH - Cl Vinyl chloride 1-Chloroethene CH2 = CH - CH2Br Allyl bromide 3-Bromopropene CH ≡ C - Cl Chloro acetylene Chloroethyne CH2I Benzyl iodide Iodophenylmethane I p-Iodotoluene 1-Iodo-4-methyl benzene or 4-Iodotoluene H3C Cl m-dichlorobenzene 1, 3-dichlorobenzene Cl 211 10.2 Nomenclature of halogen derivatives a wide variety. The hydroxyl group may be replaced by halogen atom using (a) halogen Can you recall ? acid, (b) phosphorous halide or (c) thionyl In IUPAC system of chloride. nomenclature does the a. By using halogen acid or hydrogen functional group 'halogen' appear halide (HX) : The conditions for reaction as a suffix or prefix ? of alcohol with halogen acid (HX) depend What are the trivial names of laboratory on the structure of the alcohol and particular solvents CHCl3 and CCl4 ? halogen acid used. The order of reactivity of alcohols with a given haloacid is 30>20>10. The common names of alkyl halides are (Refer to section 11.2.1 a) derived by naming the alkyl group followed R - OH + HX suitable R - X + H2O condition by the name of halogen as halide. For example, methyl iodide, tert-butyl chloride. (Alcohol) (Alkyl halide) According to IUPAC system of nomenclature Hydrogen chloride is used with zinc (Std. XI Chemistry Textbook Chapter 14, chloride (Grooves' process) for primary and section 14.4.7) alkyl halides are named as secondary alcohols, but tertiary alcohols haloalkanes. Aryl halides are named as readily react with concentrated hydrochloric haloarenes in common as well as IUPAC acid in absence of zinc chloride. system. For dihalogen derivative of an arene, anhydrous R - OH + HCl ZnCl2 R - Cl + H2O prefix o-, m-, p- are used in common name system but in IUPAC system the numerals 1,2 ; 1,3 and 1,4 respectively are used. Do you know ? Common and IUPAC names of some halogen Zinc chloride is a Lewis acid derivatives are given in Table 10.1. and consequently can coordinate with the alcohol, weakening R - O bond. Use your brain power Mixture of concentrated HCl and anhydrous Write IUPAC names of the ZnCl2 is called Lucas reagent. following. Constant boiling hydrobromic acid i. CH3 - CH - CH3 ii. CH3 - CH - CH2I (48%) is used for preparing alkyl bromides. Br CH3 Primary alkyl bromides can also be prepared by reaction with NaBr and H2SO4. Here HBr iii. CH3 - CH = CH - CH2Cl is generated in situ. NaBr, H2SO4 iv. CH3 - C ≡ C - CH2 - Br R-CH2-OH+HBr heat R-CH2-Br+H2O Br Good yield of alkyl iodides may be v. Br vi. obtained by heating alcohols with sodium or potassium iodide in 95 % phosphoric acid. Here HI is generated in situ. Br NaI/H3PO4 R - OH + HI R - I + H2O 10.3 Methods of preparation of alkyl halides 10.3.1 From alcohol : The most widely used Can you tell ? method of preparation of alkyl halide is Why phosphoric acid is preferred to replacement of hydroxyl group of an alcohol H2SO4 to prepare HI in situ ? by halogen atom. Alcohols are available in 212 b. By using phosphorous halide : An Addition of hydrogen halide to alkene alkyl halide may be prepared by action of Alkyl halides are formed on addition phosphorous halide on alcohol. Phosphorous of hydrogen halide to alkenes. Refer to Std tribromide and triiodide are usually generated XI Chemistry Textbook Chapter 15, section in situ (produced in the reaction mixture) by 15.2.4 for all the details including order of the action of red phosphorous on bromine and reactivity of HX, Markownikov rule and iodine respectively. Phosphorous pentachloride peroxide effect. reacts with alcohol to give alkyl chloride. 3R - OH + PX3 3R - X + H3PO3 Problem 10.1 : How will you obtain 1-bromo-1-methylcyclohexane from alkene? R - OH + PCl5 R - Cl + HCl + POCl3 Write possible structures of alkene and the Do you know ? reaction involved. Some times during replacement Solution : of -OH by -X, alcohols tend to undergo rearrangement. This tendency CH3 CH3 Br can be minimized by use of phosphorous halides. Straight chain primary alcohols + HBr react with phosphorous trihalide to give unrearranged alkyl halides. CH2 CH3 Br c. By using thionyl chloride : Thionyl + HBr chloride reacts with straight chain primary alcohols to give unrearranged alkyl chloride. The byproducts obtained are gases. There Use your brain power is no need to put extra efforts for its separation. Therefore this method is preferred Rewrite the following reaction for preparation of alkyl chloride. by filling the blanks : ∆ CH3 - CH = CH2 + HBr + R - OH + SOCl2 R - Cl + SO2↑+ HCl↑ (major)(minor) Can you recall ? (CH3)2C=CHCH3+HBr peroxide + Identify the products of the (major)(minor) following reactions. CH3 - CH = CH2+HBr peroxide + hν i. CH4 + Cl2 ? (major)(minor) HCl ii. CH3 - CH = CH2 ? Peroxide Do you know ? iii. CH3 - CH = CH2 + HBr ? CCl4 Alkenes form additon product, iv. CH2 = CH - CH3 + Br2 ? vicinal dihalide, with chlorine or 10.3.2 From hydrocarbon bromine usually in inert solvent like CCl4 at room temperature. Alkyl halides are formed from saturated as well as unsaturated hydrocarbons by various reactions. Halogenation of alkanes is C =C + X2 C-C not suitable for preparation of alkyl halides as a mixture of mono and poly halogen X X compounds is formed. (X = Cl, Br) 213  When toluene is brominated in presence Do you know ? of iron, a mixture of ortho and para bromo When alkenes are heated toluene is obtained. with Br2 or Cl2 at high temperature, hydrogen atom of allylic carbon is CH3 CH3 CH3 substittued with halogen atom giving Fe Br + Br2 dark + allyl halide. CH2 = CH - CH3 + Cl2 Br (o - Bromotoluene) (p-Bromotoluene) CH2 = CH - CH2Cl + HCl + HBr 10.3.3 Halogen exchange : Alkyl iodides Aromatic electrophilic substitution are prepared conveniently by treating alkyl with iodine is reversible. In this case use chlorides or bromides with sodium iodide in of HNO3/HIO4 removes HI by oxidation to methanol or acetone solution. The sodium I2, equilibrium is shifted to right and iodo bromide or sodium chloride precipitates from product is formed. F2 being highly reactive, the solution and can be separated by filtration. fluoro compounds are not prepared by this acetone method. R - Cl + NaI R - I + NaCl ↓ 10.3.5 Sandmeyer's reaction : Aryl halides The reaction is known as Finkelstein reaction. are most commonly prepared by replacement Alkyl fluorides are prepared by heating of nitrogen of diazonium salt. (For details alkyl chlorides or bromides with metal refer to Chapter 13 section 13.6). fluorides such as AgF, Hg2F2, AsF3, SbF3 etc. 10.4 Physical properties : Physical properties R - Cl + AgF R - F + AgCl ↓ of alkyl halides are considerably different The reaction is known as Swartz reaction. from those of corresponding alkanes. The boiling points of alkyl halides are determined 10.3.4 Electrophilic substitution : by polarity of the C-X bond as well as the size of halogen atoms. Can you recall ? 10.4.1 Nature of intermolecular forces: Identify the product of the Halogens (X = F, Cl, Br and I) are more following reaction. electronegative than carbon. + Cl2 anhyd. Carbon atom that carries halogen AlCl3 develops a partial positive charge while the Name the type of halide produced in halogen carries a partial negative charge. the above reaction. Thus carbon-halogen bond in alkyl halide is a What type of reactions are shown by polar covalent bond. Therefore alkyl halides benzene ? are moderately polar compounds. δ⊕ δ C X Aryl chlorides and bromides can be prepared by direct halogenation of benzene Size of the halogen atom increases from and its derivatives through electrophilic fluorine to iodine. Hence the C-X bond length substitution. It may be conveniently carried increases. The C-X bond strength decreases out in dark at ordinary temperature in presence with an increase in size of halogen. This is of suitable Lewis acid catalyst like Fe, FeCl3 because as the size of p-orbital of halogen or anhydrous AlCl3. increases the p-orbital becomes more diffused 214 and the extent of overlap with orbital of Haloalkane Boiling point (K) carbon decreases. Some typical bond lengths, CH3CH2CH2CH2Br 375 bond enthalpies and dipole moments of C-X bond are given in Table 10.2. CH3 - CH - CH2 - CH3 364 Br Table 10.2 : Bond parameters of C-X bond CH3 Bond Bond Bond Dipole CH3− C − CH3 346 length enthalpy moment Br (pm) (kJ mol-1) (debye) CH3 - F 139 452 1.847 10.4.3 Solubility : Though alkyl halides are moderately polar, they are insoluble in CH3 - Cl 178 351 1.860 water. It is due to inability of alkyl halides to CH3 - Br 193 293 1.830 form hydrogen bonds with water. Attraction CH3 - I 214 234 1.636 between alkyl halide molecules is stronger than attraction between alkyl halide and 10.4.2 Boiling point : Boiling points of alkyl water. Alkyl halides are soluble in non-polar halides are considerably higher than those of organic solvents. corresponding alkanes due to higher polarity and higher molecular mass. Within alkyl Aryl halides are also insoluble in water halides, for a given alkyl group, the boiling but soluble in organic solvents. If aryl halides point increases with increasing atomic mass are not modified by presence of any other of halogen, because magnitude of van der functional group, they show properties similar Waals force increases with increase in size to corresponding alkyl halides. The isomeric and mass of halogen. dihalobenzenes have nearly the same boiling points, but melting points of these isomers Thus boiling point of alkyl halide show variation. Melting point of para isomer decreases in the order RI > RBr > RCl > RF is quite high compared to that of ortho or meta For example, : isomer. This is because of its symmetrical structure which can easily pack closely in Haloal- CH3F CH3Cl CH3Br CH3I kane the crystal lattice. As a result intermolecular forces of attraction are stronger and therefore Boiling 194.6 248.8 276.6 315.4 greater energy is required to overcome its point (K) lattice energy. For the given halogen, boiling point rises with increasing carbon number. Cl Cl Cl For example, Cl Haloalkane Boiling point (K) Cl CH3Cl 248.8 Cl CH3CH2Cl 285.5 b.p./K 453 446 448 CH3CH2CH2Cl 320.0 m.p./K 256 249 323 CH3CH2CH2CH2Cl 351.5 For isomeric alkyl halides, boiling point decrease with increased branching as surface area decreases on branching and van der Waals forces decrease. For example : 215  Let us, now, jot down the atoms/groups Problem 10.2 Arrange the following attached to each carbon in 2 - chlorobutane. compounds in order of increasing boiling 1 2 3 4 points : bromoform, chloromethane, CH3 - CHCl - CH2 - CH3 dibromomethane, bromomethane. 2 3 4 C-1 : - H, -H, -H, -CHCl-CH2-CH3 Solution : The comparative boiling points of halogen derivatives are mainly related 1 3 4 C-2 : -H, -Cl, -CH3, -CH2-CH3 with van der Waals forces of attraction 4 2 1 which depend upon the molecular size. In C-3 : -H, -H, -CH3, -CHCl-CH3 the present case all the compounds contain 3 2 1 only one carbon. Thus the molecular size C-4 : -H, -H, -H, -CH2-CHCl-CH3 depends upon the size of halogen and number of halogen atoms present. It can be seen that the four groups bonded to C-2 are all different from each other. Thus increasing order of boiling point is, Carbon atom in a molecule which carries CH3Cl < CH3Br < CH2Br2 < CHBr3 four different groups/atoms is called chiral carbon atom. Thus, the C-2 in 2-chlorobutane 10.5 Optical isomerism in halogen is a chiral carbon. Chiral atom in a molecule derivatives : is marked with asterisk (*). For example, CH3-*CHCl-CH2-CH3. Can you recall ? When a molecule contains one chiral What is the relationship between atom, it acquires a unique property. Such a two compounds having the same molecule can not superimpose perfectly on its molecular formula? mirror image. It is called chiral molecule. A What is meant by stereoisomerism ? chiral molecule and its mirror image are not identical (see Fig. 10.1). Isomers having the same bond connectivities, that is, structural formula are called CH3 stereoisomers. Knowledge of optical CH3 isomerism, which is a kind of stereoisomerism *C will be useful to understand nucleophilic C* Cl H H Cl C2H5 substitution reactions of alkyl halides (see C2H5 mirror plane 10.6.3). 10.5.1 Chiral atom and molecular chirality CH3 CH3 Try this... C * *C H Cl Cl H Make a three - dimensional C2H5 C2H5 model of 2 - chlorobutane. Make another model which is a mirror Fig. 10.1 : Nonsuperimposable mirror images image of the first model. A chiral molecule and its mirror image Try to superimpose the two models on both have the same structural formula and, each other. of course, the same molecular formula. The Do they superimpose on each other spatial arrangement of the four different exactly ? groups around the chiral atom, however, is Comment on whether the two models different. In other words, a chiral molecule are identical or not. and its mirror image are stereoisomers of each 216 other. (Refer to Std. XI Chemistry Textbook, Chapter 14). Do you know ? Nicol prism is a special type of The relationship between a chiral prism made from pieces of calcite, molecule and its mirror image is similar to a crystalline form of CaCO3, arranged the relationship between left and right hands. in a specific manner. Nicol prism is also Therefore it is called handedness or chirality. called polarizer. (Origin : Greek word : Cheir means hand) 10.5.3 Optical activity : When an aqueous The stereoisomerism in which the isomers solution of certain organic compounds like have different spatial arrangements of groups/ sugar, lactic acid is placed in the path of atoms around a chiral atom is called optical plane polarized light, the transmitted light isomerism. The optical isomers differ from has oscillations in a different plane than each other in terms of a measurable property the original. In other words, the incident called optical activity. light undergoes rotation of its plane of To understand optical activity, we must polarization. The plane of polarization rotates know what is plane polarized light. either to the right (clockwise) or to the left (anticlockwise). This property of a substance Remember... by which it rotates plane of polarization of incident plane polarized light is known The phenomenon of optical as optical activity. The compounds which isomerism in organic compounds rotate the plane of plane polarized light are was observed first and its origin in called optically active compounds and those molecular chirality was recognized later. which do not rotate it are optically inactive compounds. Optical activity of a substance 10.5.2 Plane polarized light : An ordinary is expressed numerically in terms of optical light consists of electromagnetic waves having rotation. The angle through which a substance oscillations of electric and magnetic field in rotates the plane of plane polarized light on all possible planes perpendicular to direction passing through it is called optical rotation. of propagation of light. In accordance with the direction of optical When ordinary light is passed through rotation an optically active substance is either Nicol's prism, oscillations only in one plane dextrorotatory or laevorotatory. A compound emerge out. Such a light having oscillations which rotates the plane of plane polarized only in one plane perpendicular to direction light towards right is called dextrorotatory of propagation of light is known as plane and designated by symbol d- or by (+) polarized light. sign. A compound which rotates plane of plane polarized light towards left is called laevorotatory and designated by symbol l- or by (-) sign. Isomerism in which isomeric compounds have different optical activity is known as optical isomerism. French scientist Louis Pasteur first recognized that optical activity is associated with certain type of 3-dimensional structure of molecules. Pasteur introduced the term enantiomers for the optical isomers having equal and opposite optical rotation. 217  Figure 10.2 indicates a few objects in our Enantiomers have identical physical day to day life which exhibit superimposable properties (Such as melting point, boiling and non-superimposable mirror image points, densities, refractive index) except the relationship. sign of optical rotation. The magnitude of their optical rotation is equal but the sign of optical rotation is opposite. They have identical chemical properties except towards optically active reagent. An equimolar mixture of enantiomers (dextrorotatory and laevorotatory) is called racemic modification or racemic mixture. A racemic modification is optically inactive because optical rotation due to molecules of one enatiomer is cancelled by equal and Non superimposable Superimposable opposite optical rotation due to molecules of the other enantiomer. A racemic modification Fig. 10.2 : Superimposable and is designated as (dl) or by (±) sign. nonsuperimposable mirror image 10.5.5 Representation of configuration of molecules : Remember... Can you recall ? Optical activity is an Identify the type of following experimentally observable 3-D representation (I) and property of compounds. Chirality is (II) of a molecule and state a description of molecular structure. significance of the lines drawn. Optical activity is the consequence of W W chirality. X C X Y Molecules which contain one chiral Z atom are chiral, that is, they are Y Z nonsuperimposable on their mirror (I) (II) image. a. Fischer projection formula (cross The two non-superimposable mirror formula) : Two representations are used to image structures are called pair of represent configuration of chiral carbon and enantiomers. the 3-dimensional structure of optical isomers Enantiomers have equal and opposite on plane paper. These are (a) wedge formula optical rotation. Thus, enantiomers are and (b) Fischer projection formula (also called a kind of optical isomers. cross formula) (Std. XI Chemistry Textbook Chapter 14 section 14.2.3). 10.5.4 Enantiomers : The optical isomers Cl Chiral carbon Cl Bonds below the plane which are non-superimposable mirror image I Br I C Br of each other are called enantiomers or enantiomorphs or optical antipodes. For Bonds above H the plane H example, 2 - chlorobutane exists as a pair of Fischer projection Convention of vertical enantiomers (Fig. 10.1). and horizontal lines Fig. 10.3 Fischer projection formula 218 b. Wedge formula : When a tetrahedral Can you recall ? carbon is imagined to be present in the plane What is meant by substitution of paper all the four bonds at this carbon reaction ? cannot lie in the same plane. The bonds in the plane of paper are represented by normal Can you identify substitution reaction lines, the bonds projecting above the plane from the following ? of paper are represented by solid wedges (i) CH3 - CH2 - OH + HCl ZnCl2 (or simply by bold lines) while bonds going CH3 - CH2 - Cl + H2O below the plane of paper are represented by broken wedges (or simply by broken lines). (ii) CH2 = CH2 + HI CH3 - CH2 - I Br Is the carbon carrying halogen in alkyl below the plane In the plane C halide, an electrophilic or a nucleophilic I Cl centre ? H Above the plane hybridization of that carbon the reaction is Try this... called substitution reaction. The C-X bond in alkyl halides is a polar covalent bond 1. Draw structures of enantiomers and the carbon in C-X bond is positively of lactic acid (CH3-CH-COOH) polarized. In other words, the C-X carbon is OH an electrophilic centre. It has, therefore, a using Fischer projection formulae. tendency to react with a nucleophile. (Refer to 2. Draw structures of enantiomers of Std. XI Chemistry Textbook Chapter 14.) Alkyl 2-bromobutane using wedge formula. halides react with a variety of nucleophiles 10.6 Chemical properties : to give nucleophilic substitution reactions (SN). The reaction is represented in general 10.6.1 Laboratory test of haloalkanes : form as shown below. Haloalkanes are of neutral type in aqueous δ⊕ δ medium. On warming with aqueous sodium Nu + − C − X − C − Nu + X or potassium hydroxide the covalently bonded halogen in haloalkane is converted to halide When a substrate reacts fast it is said to ion. be reactive. The reactivity of alkyl halides ∆ in SN reaction depends upon two factors, R - X + OH R - OH + X namely, the substitution state (10, 20 or 30) When this reaction mixture is acidified of the carbon and the nature of the halogen. by adding dilute nitric acid and silver nitrate The order of reactivity influenced by these solution is added a precipitate of silver halide two factors is as shown below. is formed which confirms presence of halogen tertiary alkyl halide (30) > secondary alkyl in the original organic compound. halide (20) >primary alkyl halide (10) and Ag⊕ (aq) + X (aq) AgX↓ (s) R - I > R - Br > R - Cl Examples of some important nucleophilic 10.6.2 Nucleophilic substitution reactions of substitution reactions of alkyl halides are haloalkanes : shown in Table 10.3. When a group bonded to a carbon in a substrate is replaced by another group to get a product with no change in state of 219  10.3 Nucleophilic substitution reactions of alkyl halides Sr. No. Alkyl halide Reagent Substitution product 1. R-X + NaOH(aq) ∆ R - OH + NaX (or KOH) (alcohol) (or KX) ⊕ NaOR' ∆ R - O - R' + NaX 2. R-X + (sodium alkoxide) (ether) O ⊕ O 3. R-X + R' - C - OAg ∆ R' - C - OR + AgX ↓ (silver carboxylate) (ester) ∆ NH3(alc.) R - NH2 + HX 4. R-X + pressure (excess) (primary amine) KCN (alc.) ∆ R - CN + KX 5. R-X + (nitrile)(alkyl cyanide) AgCN (alc.) ∆ R-N C + AgX ↓ 6. R-X + (isocyanide) ⊕ KO - N = O R-O-N=O + KX 7. R-X + (potassium nitrite) (alkyl nitrite) Ag - O - N = O ⊕ O 8. R-X + R N + AgX ↓ (silver nitrite) O (nitroalkane) Can you tell ? Do you know ? Alkyl halides when treated with Cyanide ion is capable of alcoholic solution of silver nitrite give attacking through more than one nitroalkanes whereas with sodium nitrite site (atom). they give alkyl nitrites Explain. C≡N C=N 10.6.3 Mechanism of SN reaction : Such nucleophiles are called ambident Can you recall ? nucleophiles. KCN is predominantly ionic What is meant by order and (K⊕C ≡ N) and provides cyanide ions. Both molecularity of a reaction ? carbon and nitrogen are capable of donating electron pair. C-C Bond being stronger than What is meant by mechanism of C-N bond, attack occurs through carbon atom chemical reaction ? of cyanide group forming alkyl cyanides as It can be seen from the Table 10.3 that in major product. However AgCN (Ag-C ≡ N) nucleophilic substitution reactions of alkyl is mainly covalent compound and nitrogen halides the halogen atom gets detached from is free to donate pair of electron. Hence the carbon and a new bond is formed between attack occurs through nitrogen resulting in that electrophilic carbon and nucleophile. formation of isocyanide. The covalently bonded halogen is converted Another ambident nucleophile is nitrite into halide ion (X ). It means that the two ion, which can attack through ‘O’ or ‘N’. electrons constituting the original covalent bond are carried away by the halogen along O-N=O with it. The halogen atom of alkyl halide is, therefore, called ‘leaving group’ in the 220  [ [ context of this reaction. Leaving group is the group which leaves the carbon by taking H 1 H 1 2 2 away the bond pair of electrons. The substrate HO C Br HO C Br undergoes two changes during a SN reaction. H H H The original C-X bond undergoes heterolysis H Transition state and a new bond is formed between the carbon (T.S.) and the nucleophile using two electrons of the H nucleophile. These changes may occur in one or more steps. The description regarding the HO C + Br sequence and the way in which these two H changes take place in SN reaction is called H mechanism of SN reaction. The mechanism is Fig. 10.4 : SN2 mechanism deduced from the results of study of kinetics Salient features of SN2 mechanism : of SN reactions. Two mechanisms are observed i. Single step mechanism with simultaneous in various SN reactions. These are denoted as bond breaking and bond forming. SN1 and SN2 mechanisms. ii. Backside attack of nucleophile : The a. SN2 Mechanism : The reaction between nucleophile attacks the carbon undergoing methyl bromide and hydroxide ion to give substitution from the side opposite to that methanol follows a second order kinetics, of the leaving group. This is to avoid steric that is, the rate of this reaction depends on repulsion (repulsion due to bulkyness of concentration of two reacting species, namely, the groups) and electrostatic repulsion methyl bromide and hydroxide. Hence it is between the incoming nucleophile and called subtitution nucleophilic bimolecular, the leaving group. SN2. iii. In the transition state (T.S.) the nucleophile CH3Br + OH CH3OH + Br and leaving groups are bonded to the rate = k [CH3Br] [OH ] carbon with partial bonds and carry partial negative charge. (Thus, the total Rate of a chemical reaction is influenced by negative charge is diffused.) the chemical species taking part in the slowest step of its mechanism. In the above reaction iv. The T.S. contains pentacoordinate only two reactants are present and both are carbon having three σ (sigma) bonds in found to influence the rate of the reaction. one plane making bond angles of 1200 This means that the reaction is a single step with each other and two partial covalent reaction which can also be called the slow bonds along a line perpendicular to this step. This further implies that the two changes, plane. namely, bond breaking and bond forming at v. When SN2 reaction is brought about at the carbon take place simultaneously. This chiral carbon (in an optically active SN2 mechanism is represented as shown in substrate), the product is found to have Fig. 10.4. opposite configuration compared to that of the substrate. In other words, SN2 reaction is found to proceed with inversion of configuration. This is like flipping of an umbrella (See Fig. 10.4). It is known as Walden inversion. The inversion in configuration is the result of backside attack of the nucleophile. 221 b. SN1 Mechanism : The reaction between iv. When SN1 reaction is carried out at chiral tert-butyl bromide and hydroxide ion to give carbon in an optically active substrate, tert-butyl alcohol follows a first-order kinetics, the product formed is nearly racemic. that is the rate of this reaction depends on This indicates that SN1 reaction proceeds concentration of only one species, which is mainly with racemization. This means the substrate molecule, tert-butyl bromide. both the enantiomers of product are formed Hence it is called substitution nucelophilic in almost equal amount. Racemization in unimolecular, SN1. SN1 reaction is the result of formation of planar carbocation intermediate (Fig. CH3 CH3 10.5). Nucleophile can attack planar CH3− C − Br + OH CH3− C − OH + Br carbocation from either side which results CH3 CH3 in formation of both the enantiomers of rate = k [(CH3)3CBr] the product. It can be seen in this reaction that Use your brain power concentration of only substrate appears in the rate equation; concentration of the Draw the Fischer projection nucleophile does not influence the reaction formulae of two products rate. In other words, tert-butyl bromide reacts obtained when compound (A) reacts with hydroxide by a two step mechanism. with OH by SN1 mechanism. In the slow step C-X bond in the substrate C2H5 undergoes heterolysis and in the subsequent H3C− C − Br (A) fast step the nucleophile uses its electron pair to form a new bond with the carbon n-C3H7 undergoing change. This SN1 mechanism is Draw the Fischer projection formula of represented as shown in Fig. 10.5. the product formed when compound (B) Step I reacts with OH by SN2 mechanism. CH3 (CH3)3C - Br slow ⊕ CH3 C + Br H− C − Cl (B) H 3C CH3 C2H5 (carbocation intermediate) 10.6.4 Factors influencing SN1 and SN2 Step II mechanism : CH3 a. Nature of substrate : SN2 : The T.S. of C⊕ + OH (CH3)3C - OH SN2 mechanism is pentacoordinate and thus H3C CH3 crowded (See Fig. 10.4). As a result SN2 mechanism is favoured in primary halides Fig. 10.5 : SN1 mechanism and least favoured in tertiary halides. Salient features of SN1 mechanism : SN1 : A planar carbocation intermediate i. Two step mechanism. is formed in SN1 reaction. It has no steric ii. Heterolyis of C-X bond in the slow crowding. Bulky alkyl groups can be easily and reversible first step to form planar accommodated in planar carbocation See carbocation intermediate. (Fig. 10.5). As a result SN1 mechanism is iii. Attack of the nucleophile on the most favoured in tertiary halides and least carbocation intermediate in the fast favoured in primary halides. (Formation of second step to form the product. planar carbocation intermediate results in a 222  H H H H H H H H H 1 H 1 1 C 1 1 C 1 1 C 1 2 2 2 2 2 2 2 2 (a) Nu C X Nu C X Nu C X Nu C X H H H H H C H H C C H H H H H H H crowding and destabilization increases H H H αH H αH H H H H C (b) C C C⊕ C⊕ C⊕ C⊕ H H α H Cα C H H H H Cα H H H H H H H steric relief, stabilization by +I and hyperconjugation of α - hydrogens increases Fig. 10.6 : Influence of substrate in SN1 and SN2 (a) Transition states (T.S.) in SN2 (b) Carbocation intermediates in SN1 relief from steric crowding present in the Problem 10.4 : Primary allylic and tertiary halide substrate). primary benzylic halides show higher Secondly the carbocation intermediate is reactivity by SN1 mechanism than stabilized by +I effect of alkyl substituents other primary alkyl halides. Explain. and also by hyperconjugation effect of alkyl Solution : SN1 reaction involves substituents containing α-hydrogens. As a formation of carbocation result, SN1 mechanism is most favoured intermediate. The allylic and benzylic in tertiary halides and least favoured in carbocation intermediate formed are primary halides. This can be represented resonance stabilized, and hence SN1 diagramatically as shown below. mechanism is favoured. SN1 rate increases ⊕ ⊕ CH2 = CH - CH2 CH2 - CH = CH2 CH3 - X 10 20 30 Resonance stabilization of allylic carbocation SN2 rate increases ⊕ Tertiary halides undergo nucleophilic CH2 CH2 CH2 substitution by SN1 mechanism while primary ⊕ ⊕ halides follow SN2 mechanism. Secondary ⊕ halides react by either of the mechanism or ⊕ CH2 CH2 by mixed mechanism depending upon the exact conditions. Resonance stabilization of benzylic carbocation 223